Back To CourseUExcel Microbiology: Study Guide & Test Prep
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
One of the precautions given to expectant mothers is that they absolutely cannot clean out a cat litter box. When I was younger, I used to think that it was a fringe benefit to being pregnant, an excuse for Mom to take it easy and not have to work so much. In reality, that litter box could house a potentially devastating illness that pregnant women are especially susceptible to.
The disease-causing organism that sweet little Whiskers carried into the house and deposited in the litter box is Toxoplasma gondii. T. gondii is a single-celled coccidian parasite that causes toxoplasmosis in humans. It is estimated that 30% to 60% of people will test positive for T. gondii even though most of them never showed any clinical signs of the infection or are even aware that they have been exposed. The only symptoms are usually a mild fever and swollen lymph nodes. You would probably guess you had a mild flu and forget about it in a couple of days.
Believe it or not, parasitology experts consider T. gondii to be the most common parasite infecting humans and other warm-blooded vertebrates but one of the rarest when it comes to causing actual disease.
T. gondii is an intracellular parasite. When it comes in contact with a host cell, it attaches to the cell membrane, makes a small ring-like structure and pulls the membrane up over itself. Just imagine pulling a sock on over your foot. Muscle and nerve cells are common tissues infected but any cell is fair game (with the somewhat odd exception of erythrocytes).
Let's dive a little deeper into the life cycle of T. gondii. We'll start our discussion with a cat, the definitive host of T. gondii. The definitive host is simply the organism that houses the adult, sexually mature form of the parasite. Let's say our cat has ingested a T. gondii cyst, or a resistant form. The cyst passes into the small intestine and ruptures. The larval tachyzoites, or the rapidly dividing stage, emerge and invade the cat's tissues via the bloodstream, seeming to prefer the lungs and eyes. They mature rapidly, then reproduce sexually, resulting in oocyst production. Oocysts house the immature, developing parasite, a little like an egg. The cat will shed the oocysts in its feces for only one to two weeks during the lifetime of the cat and only after the first infection with T. gondii.
The shed oocysts are not capable of producing illness for at least 24 hours and sometimes as long as five days. If an active oocyst is eaten by an unexposed cat, the parasite can mature and reproduce. If the oocyst is eaten by another animal, like a mouse or human, another cycle begins. The mouse, for example, is an intermediate host, which houses the immature, asexual stage of the parasite. In this case, bradyzoites, or the slowly dividing stage, will release into the intestine and spread to body tissues while remaining immature. A cell that lands in a mouse muscle tissue, for example, will form a cyst. In this cyst, the single parasite will divide asexually, filling the space with clones of itself. This structure can then remain dormant for the life of the intermediate host.
The intermediate host mouse may never know it has been infected and has cysts in its muscle tissue. If an uninfected cat eats the mouse muscle tissue containing the cyst, immature T. gondii will burst forth and infect the cat as a new definitive host.
So how does a person become infected with T. gondii and how can it be prevented?
The most sensationalized, yet probably the least common source, is directly from your pet cat. The oocysts that are passed in the cat's feces are potentially infectious. Also, oocysts clinging to the cat's paws after it uses the litter box can be deposited on household surfaces. Accidental ingestion can lead to infection. There are a couple of key points to make here. The oocysts are not infectious for at least 24 hours, so daily litter changes are usually enough to eliminate this source. Also, the cat will only shed oocysts for one to two weeks in its entire lifetime. You would have to be very unlucky to coordinate your accidental fecal ingestion with that time period.
The next source of exposure is by ingesting contaminated soil. Outdoor cats bury their waste and prefer the soft, disturbed soil of gardens and sandboxes. An infected cat can deposit infectious oocysts in these areas, leading to human exposure. This threat can be eliminated by wearing gloves while gardening, washing your hands thoroughly afterwards, and covering sand boxes when they are not in use.
The most common source of infection is eating undercooked meat. Remember, T. gondii can encyst in muscle. This could include livestock, like cattle or pigs, as well. If you enjoy rare steaks, be advised that they could contain viable cysts. No need to turn vegetarian yet, though. Simply cooking it to 140 degrees Fahrenheit (60 degrees Celsius) will kill all cysts. Since there is no vaccine available, these preventative measures are the best course of action.
I mentioned that 30% to 60% of people have been exposed to T. gondii and never know it. In these people, the parasites form cysts in various tissues, like the brain or muscle. But there are two groups of people that need to be extra vigilant.
The first is pregnant women. T. gondii cells are able to infect babies in utero transplacentally, or through the placenta. If a woman ingests active cysts or oocysts during the first trimester, tachyzoites can migrate through the placenta, and the fetus can develop lesions on the immature organs, which can lead to severe birth defects or spontaneous abortions. If the infection occurs during the third trimester, the baby can be born with mental or physical defects, like blindness, reduced mental ability, seizures, or grossly swollen organs. Both conditions are devastating to the health of the child.
The second susceptible group is people with immunodeficiency diseases, like AIDS. Initial infections can be severe, invading brain and organ tissues, causing swelling and inflammation. Also, cysts present from a previous infection can reactivate and cause similar symptoms.
Treatment for active infections is usually only needed in patients with compromised immune systems. A combination of a sulfonamide antibiotic and the anti-protozoal pyrimethamine is the most effective treatment, but it can take up to a year to completely clear the infection.
These drugs can have severe side effects in fetuses and are only used in very dire infections of pregnant women. The antibiotic spiramycin has proven effective at preventing transmission to the fetus.
Let's review. Toxoplasmosis is a protozoal disease caused by Toxoplasma gondii. The parasite can only reproduce in the cat, its definitive host. T. gondii can encyst in other warm-blooded animals, known as intermediate hosts, but can't reproduce sexually.
Most people that get toxoplasmosis never show symptoms, but pregnant woman and AIDS patients are at risk. Severe brain and organ problems can occur if the immune system is unable to fight the parasite or if the developing fetus is exposed.
People commonly get T. gondii by eating cysts in undercooked meat. Accidental ingestion of cysts in soil or exposure from pet cats are also sources of infection, though less so than meat. Transplacental infection occurs when a mother ingests cysts that pass to a fetus through the placenta. There are drugs that can kill the parasite, but treatment can take up to a year.
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Back To CourseUExcel Microbiology: Study Guide & Test Prep
28 chapters | 241 lessons